Daylight savings time, day length and plants

November 6, 2026

Patricia Hanbidge, Lead Horticulturist, Orchid Horticulture

Key points from this story:

• Plants use internal circadian rhythms
• Photoperiodism regulates flowering
• Short-day plants measure night length
• Poinsettias need uninterrupted darkness
• Long-day plants can be dark-dominant
• Day-neutral plants bloom when mature

Last week we introduced a focus on day length, daylight savings time and plants. It was mentioned that plants do not read human clocks. Instead, plants use an internal circadian rhythm driven by actual sunlight, temperature shifts and natural day length (photoperiods) rather than time zones or man-made clock adjustments. Plants have a biological clock that runs on a 24-hour cycle using light-sensing proteins to coordinate their activities like photosynthesis and metabolism.

Photoperiodism in plants is when plants use the length of light and darkness to regulate flowering and other processes. Photoperiodism acts without any input from the ambient temperature or the weather. It is strictly a light/dark kind of thing. However, photoperiodism is dependent on the ability of the plant to sense light. Plants have photoreceptors or cells that contain a light absorbing pigment. When triggered these cells will alter specific proteins within the plant which cause changes in hormone production, gene expression and in growth. What this means in simpler terms is that photoperiodism is a response of plants to light exposure which then causes change within the plant.

We talked about simple short-day and long-day plants, but this week, I would like to dig a little bit deeper into the science behind some of the activities happening in the plants that are around us. Although we classify plants as short- or long-day plants, there are some short-day plants that actually measure the length of the night that consists of continuous darkness in order to trigger the bloom. Thus, many of our short-day plants will flower when the day is short and the night is long, but will not flower if the long night is interrupted by a brief period of light! A very good example of this characteristic is the poinsettia which will not trigger the formation of the bloom if the darkness period is interrupted.

When looking at long-day plants, some will measure the length of the night, and require that dark period to be shorter than or equal to a critical length of darkness. Those long-day plants measuring the night period are considered to be "dark-dominant" as it is the dark period that is important to flowering. Many of the long-day plants will instead measure the length of the day, rather than the night and these plants are considered to be "light-dominant".

Just to muddy the waters a bit more, we also have day-neutral plants that are not regulated by day length at all. These plants bloom when they are mature enough to do so. Plants that are day-neutral include cucumbers, tomatoes and corn. One other item to think about is that plants are actually very complex. Many of our outdoor plants use a variety of triggers to prepare for the end of the growing season and the ensuing winter deep freeze. Tuber formation in potatoes and trees that are in the process of preparing for winter are also mainly controlled by the length of daylight but other factors will also play a role.

As you can see, plants are very complex and there are many factors involved in them just going about their daily work. I hope that these articles have helped you to understand just a little bit more about how plants live and why they do not recognize daylight savings time.